1. Field of the Invention
The present invention relates to the provision and use of a programmable boring and contouring apparatus or xe2x80x9chead assemblyxe2x80x9d for performing three-dimensional machining operations such as boring, contouring, threading, turning, facing and the like, using a head assembly that is rotated about a primary rotation axis (PRA) of a machining center, wherein the head assembly has a radially movable tool holder for supporting a cutting tool at a radial offset distance (ROD) from the PRA which is controlled, at least in part, through the use of a micro-controller and a servo motor that are components of the head assembly itself. More particularly, the present invention relates to a boring and contouring apparatus or xe2x80x9chead assemblyxe2x80x9d having on-board logic 1) for being mounted on the rotatable spindle of a machining center such as a boring machine, 2) for being rotated about a primary axis of rotation (PRA) of the spindle, and 3) for supporting a cutting tool such as a boring bar at a controllable radial offset distance (ROD) from the PRA, wherein the ROD is adjusted by the head assembly as the head assembly is advanced along the PRA by the machining center, so that a bore or other desired type of workpiece formation is formed having one or more turned surfaces that differ in diameter as a function of their location along the PRA or vice versa. Features of the invention reside not only in the apparatus and but also in methods by which turned surfaces of varying diameter are formed, and in safety features that obtain through providing the apparatus with an electronically readable identification code that can be checked by control logic external to the head assembly to ensure that a correctly programmed head assembly carrying a correct cutting tool is being installed on a machining center to perform desired functions.
2. Prior Art
So-called xe2x80x9cstandard machining centersxe2x80x9d are known that each utilize a spindle which is rotatable (relative to a workpiece on which cutting operations are to be performed) about a spindle rotation axis (referred to herein by the term xe2x80x9cprimary rotation axisxe2x80x9d or by the abbreviation xe2x80x9cPRAxe2x80x9d), wherein the spindle is used to support a tool head or xe2x80x9chead assemblyxe2x80x9d that rotates (together with the spindle on which it is mounted) about the PRA to bring a cutting tool that is carried by the head assembly into engagement with the workpiece to perform boring or other turning type cutting operations that center about the PRA.
Head assemblies utilized for such purposes typically are provided with tool holders that are manually adjusted to radially position (at a selected radial offset distance from the PRA, referred to herein by the abbreviation xe2x80x9cRODxe2x80x9d) such cutting tools as are mounted on their tool holders. Head assemblies having manually adjustable tool holders (i.e., the ROD of the tool holder is manually adjustable) often are used to bore holes of selected diameters in workpieces or to form turned surfaces with features having diameters that vary as functions of the position of these features along the associated PRA, or vice versa. Usually these manually adjustable head assemblies require that relative rotation of the head assembly and the workpiece be stopped when adjustments (i.e., ROD adjustments) are to be made as by manually repositioning the tool holders on the head assemblies.
Head assemblies also are known that incorporate an ability to effect ROD adjustments of their tool holders without stopping relative rotation of the tool holders and the workpieces. However, many of these so-called xe2x80x9ceasier to adjustxe2x80x9d head assemblies have one or more associated drawbacks that have left a long-standing need for an improved boring and contouring apparatus that offers a radially adjustable tool holder that has the ability to coordinate the radial and axial positioning of cutting tools in a simple and effective manner to provide smoothly contoured workpiece surfaces of desired configuration.
One common drawback of present-day head assemblies that have tool holders with ROD adjustments that can be made during tool rotation is that they are not well suited for use with standard machining centers unless modifications are made to provide some means for transmitting control signals to the rotating head assembly to cause the cutting tool it carries to be properly ROD positioned. One approach that has been taken is to replace the normally solid spindles of standard machining centers with hollow spindles, through which mechanical movements (i.e., xe2x80x9cmechanical control signalsxe2x80x9d) can be provided to rotating head assemblies to cause their tool holders to effect proper ROD adjustments of cutting tools as cutting operations proceed. Another approach that has been taken is to provide standard machining centers with mechanical apparatus located to one side of their rotating head assemblies for transmitting control motions (i.e., xe2x80x9cmechanical control signalsxe2x80x9d) to the rotating head assemblies to carry out needed ROD adjustments mechanically. Still other approaches have called for the use of electrical contacts to transmit electrical control signals, or the use of transmitters and receivers of such non-mechanical, non-electrical control signals such as infrared light, to the rotating head assemblies so the head assemblies will coordinate the ROD positioning of cutting tools (that are being rotated by the head assemblies as the head assemblies are rotated by the machining centers) with the axial positions of these tools along the PRAxe2x80x94so that desired contours and profiles can be generated as cutting operations proceed.
If a standard machining center that has a capability to control the rotation and axial positioning along the PRA of a head assembly is to be provided with the added capability to control the ROD positioning of a cutting tool that is carried by the head assembly, this necessitates that some means be provided to coordinate ROD movements of a tool holder of the head assembly with the movements of other components of the machining center. Stated in another way, the addition of a radial tool positioning capacity to a machining center that does not have such a capacity has typically required that the machining center""s numerical control system be modified or replaced, otherwise the required coordination of movements cannot be achieved. Making such modifications and replacements requires considerable technical skill and a good working knowledge of numerical controls, is expensive, and risks the introduction of inaccuracies and errors inasmuch as numeric control system components and their specifications must be matched to the particular servo motors and gear sizes and other features of the components that are used. In many instances, it may not be possible to retrofit a machining center with a new numerical control system that will provide radial cutting tool positioning control (i.e., that will control the ROD of a cutting tool which calls for the machining center to provide tool positioning control along what amounts to an xe2x80x9cadded axisxe2x80x9d) that is properly coordinated with other cutting tool movements (such as rotation about and axial movement along the PRA of the spindle).
Many of these proposed modifications and/or additions result in the provision of non-standard apparatus that is not interchangeable from one work station to another, and that is not well suited for use with automatic tool changing equipment that may be required in order to enable a machining center to perform sequential operations with efficiency. Some of the proposed additions and/or modifications provide non-standard structure that is so bulky, heavy and/or dynamically out of balance as to be unusable with smaller machining centers. In many instances, the additions and modifications that are required are found to compromise machining accuracy.
A deficiency of most previously proposed head assemblies has been their inability to automatically advance the radial position (i.e., the ROD) of a cutting tool during tool rotation, or to take other needed steps to accommodate or respond to factors that are encountered in a working environment, such as vibration, wear and backlash. Vibration may be encountered because the voltage (or other characteristics) of electrical control signals being fed to servo motors that control the positioning of components is too low or otherwise needs to be altered (for example, by making a needed voltage adjustment) to optimize servo motor performance under conditions that are encountered in carrying out a particular cutting operation. Backlash (in the form of xe2x80x9cplayxe2x80x9d or looseness in drive system components that couple servo motors to tool holders to position the tool holders) may be present either due to the design of the drive system being used, or due to drive component wear that may occur with extended use. Compensation often needs to be made for such backlash if workpieces are to be machined with desired accuracy.
Still another deficiency has been the failure to incorporate safety features to prevent machining center operation when an incorrect head assembly or a head assembly carrying an incorrect cutting tool is installed thereonxe2x80x94a problem that easily can cause workpiece and/or apparatus damage as well as operator injuryxe2x80x94a problem that necessitates that communication take place between a machining center and the head assemblies that are installed thereon, so that dangerous mismatches are prevented.
Other deficiencies encountered with previously proposed head assemblies that attempt to provide ROD adjustment of a cutting tool during rotation are associated with the physical size, complexity and cost of the proposed head assemblies. While it is known to provide head assemblies with counterweights that can be positioned to compensate at least to some degree for dynamic imbalance due to the radial offset positioning of a cutting tool and its tool holder, some proposed ROD-adjustable head assembly proposals eliminate the provision of counterweights because the head assemblies themselves are already too bulky in size, to complex in configuration, and/or too costly to permit the addition of counterweights. Compactness and simplicity of design have not been well achieved with many prior ROD adjustable head assembly proposals, high cost has prohibited wide implementation of prior head assembly proposals, and undue complexity of design has brought with it the attendant curse of operational unreliability and limited service life between xe2x80x9cdown timesxe2x80x9d when maintenance by trained technicians to replace deteriorated parts or to rebuild the head assemblies has proven necessary.
The present invention addresses the foregoing and other needs and drawbacks of the prior art by providing a well designed boring and contouring apparatus or xe2x80x9chead assemblyxe2x80x9d that has an on-board micro-controller providing on-board logic that assists in carrying out such activities as 1) coordinating the radial positioning of the tool holder of the head assembly (with respect to a PRA of the head assembly) with other activities including the axial positioning (along the PRA) of the head assembly, 2) compensating for vibration, wear and backlash, and 3) communicating with an xe2x80x9coff boardxe2x80x9d or remotely located controller (referred to from time to time herein as a xe2x80x9ccoordinating controllerxe2x80x9d) to prevent damage and injury that can result if use is mistakenly attempted of an incorrectly programmed head assembly and/or a head assembly carrying an incorrect tool.
One aspect of the preferred practice of the present invention centers about the provision of a head assembly that incorporates a micro-controller, a servo motor and sensors to independently control the radial positioning (i.e., the xe2x80x9cRODxe2x80x9d) of a cutting tool mounted on the tool holder of the head assembly for rotation about the primary rotation axis (i.e., the xe2x80x9cPRAxe2x80x9d) of the head assembly. This arrangement provides a means of augmenting the functionality of machining centers that lack the capability to alter the radial position (i.e., to adjust tool offset or xe2x80x9cRODxe2x80x9d relative to the primary rotation axis or xe2x80x9cPRAxe2x80x9d) of a rotating cutting tool, and/or the capability to coordinate the radial positioning of a tool holder with other real-time activities.
A significant aspect of the preferred practice of the present invention resides not only in the capability that is provided to independently control the radial positioning of a cutting tool carried by a tool holder of the head assembly, but perhaps more importantly the capability that is provided to link the radial positioning of the cutting tool to sensed positioning of the head assembly as the head assembly is translated along the primary rotation axis by the machining center that supports and rotates the head assembly. In accordance with preferred practice, control logic associated with the head assembly is provided with a program of instructions that defines for the head assembly a plurality of radial offset distances of the cutting tool that are to correspond to a set of axial positions of the cutting tool so that contours of a wide variety of configurations can be formed on a workpiece that is engaged by the cutting tool as the cutting tool is simultaneously moved axially along the primary axis of rotation by the machining center and moved radially by the head assembly. Smooth contours can be generated because radial offset positioning of the cutting tool is achieved in real time in response to sensed real time axial positionings of the cutting tool, with radial offset positioning being controlled as a function of sensed axial positioning as controlled by a program of instructions provided to control logic associated with the head assembly.
Still another aspect of preferred practice resides in providing a head assembly with a capability to automatically compensate for vibration, wear and backlash in radially positioning a rotating cutting tool. For example, the on-board micro-controller circuitry utilized in the preferred practice of the present invention includes an accelerometer chip that provides an output that varies with the character of sensed vibrationxe2x80x94an output that may be is utilized by the on-board micro-controller, for example, to alter control signals that are being supplied to a servo motor that positions the cutting tool, which can be altered to minimize tool chatter that often is the cause of problematic vibration.
To compensate for backlash and wear in drive components that connect the servo motor to a tool slide of the head assembly that positions the tool holder, the system of the present invention preferably includes a pair of sensors located at opposite ends of the drive train. One of the sensors monitors the rotation of the drive shaft of the servo motor so that the on-board micro controller logic circuitry knows what the servo motor is doing in an effort to properly position the tool holder. The other of the sensors monitors the actual movements and positioning of the tool holder so that the on-board micro-controller also knows what actually is happening as efforts are made by the servo motor to position the tool holder. By comparing what the motor thinks it is doing with what actually is happening, the control signal being fed to the servo motor can be altered from xe2x80x9cthat which was expected to be needed to accomplish the intended resultxe2x80x9d to conform with xe2x80x9cthat which actually is needed under the circumstances encountered in the present real-time situation to achieve the needed resultxe2x80x9d xe2x80x94which is another way of saying that backlash (i.e., play or looseness) and wear in the drive system, together with other real-time factors, are taken into account and overcome by the system of the present invention.
Another aspect of preferred practice resides in the addition to a head assembly of safety features that aid in preventing inappropriate machine operationxe2x80x94such as may occur if an incorrectly programmed head assembly carrying an inappropriate tool is installed on the spindle of a machining center and operation is undertaken. In accordance with preferred practice, a head assembly that is programmed to perform a particular function on a particular machining center may be prevented from being installed on and operated by another machining center.
A significant feature that obtains with the head assembly design that embodies the preferred practice of the present invention is simplicity and compactnessxe2x80x94i.e., a design that utilizes a novel and reliable arrangement of components that are combined in a cleverly nested manner that permits rugged drive components to nest and protectively shield more delicate electrical and electronic components, yielding a very compact, lightweight head assembly. Drive train components including a servo motor, a gear reducer and an encoder that monitors the servo motor are nested at the center of the head assembly, with the on-board micro-controller and other attendant circuitry carried on a circuit board that wraps compactly about the periphery of the gear reducer, and with the much heavier drive components that transmit torque from the spindle of the machining center to the cutting tool taking a tubular form that defines the outer diameter of the head assembly and that protectively surrounds the servo drive train components and the delicate circuitry of the head assembly.
A feature that also obtains with the preferred practice of the present invention is the inclusion of a set of counterweights that move automatically in directions opposite to the directions of movement of the tool slide that carries the tool holder and the head assembly. The compactness and simplicity of other components of the head assembly, and the compactness and simplicity of the counterweight system that is employed, make it possible for the head assembly to include counterweights that move automatically in directions opposite to movements of the tool slide to compensate for dynamic imbalance. The compactness of the counterweight system, and the lightness and compactness of the other components of the head assembly permits the inclusion of the counterweight system in the head assembly without adding unduly to the weight, size and complexity of the head assembly.
A desirable objective that is achieved with the preferred practice of the present invention is the provision of a head assembly that performs such functions as are outlined above while requiring the attachment of no new or unusual hardware to the spindle of a machining center. Head assemblies that embody the preferred practice of the present invention are shaped, sized and appropriately configured to enable them to be attached or removed from the spindle of a machining center by standard automatic tool changer equipment commonly used with machining centers.
Another desirable objective that is achieved with the preferred practice of the present invention is the provision of a head assembly that adds the aforementioned functions and features to the capability of a machining center without requiring modifications to or replacement of the control system that already is being used by the machining center. Augmenting the operation of the on-board micro-controller of the head assembly is an xe2x80x9coff-boardxe2x80x9d or remotely located controller (a so-called xe2x80x9ccoordinating controllerxe2x80x9d) that preferably is coupled to the existing control system of the machining center (the off-board controller can be coupled to existing control systems of a variety of types) to permit the control logic that is associated with the head assembly to intercept signals transmitted by the machining center""s existing control system to one or more of the machining center""s existing servo motorsxe2x80x94so that intercepted information can be utilized by the control system of the present invention to correctly radially position a cutting tool relative to the axis of rotation (i.e., to effect ROD positioning relative to the PRA).
In preferred practice, the on-board and coordinating controllers (of the control logic that is associated with the head assembly) communicate with each other and with the control system of the machining center by means of asynchronous digital serial interfaces of the variety commonly used by the control systems typically found on present-day machining centers so as to allow commands and parameters to be sent to and among, and status received from and among these controllers to allow direct control and programming of the functions of the controllers to effect the desired operation of the head assembly, with real-time direct control of ROD positioning being provided together with monitoring of what is happening.